particule injection

I am injecting some particules . Two possibility in my model (at least what i want) be trapped on one surface of the geometry or escaped by the outlet. But more than 50% of my particules are incomplete. When i am tracking them one by one, i dont see any incomplete. But i should have 50% chance to see at leats one. What could be the cause of that please. I increase the number of step but doesnt change anything.... thanks